Girls in girl-boy twin pairs may be a bit heavier

NEW YORK (Reuters Health) - Girls who have a male twin seem more likely to put on a few extra pounds as they get older, suggests a new study.

Previous studies have suggested that women with a male twin have higher levels of aggressiveness and rule-breaking behavior, as well as a lower risk of eating disorders.

For this study, published in the International Journal of Obesity, Dr. Elias Eriksson and his colleagues at the University of Gothenburg in Sweden consulted a database of Swedish twins born between 1886 and 1958. During 1998 through 2003, researchers collected information from about 17,500 females who had a twin. Slightly less than half of those women had a male twin, and the others had a female fraternal twin.

In phone interviews, women answered health-related questions including their height and weight and whether they had diabetes or a condition in which levels of fats and cholesterol in the blood are too high.

At the time of the phone interview, women were between 42 and 103 years old.

The actual difference in weight was literally just a few pounds: On average, women with a male twin weighed almost 147 pounds, compared to about 145 pounds for women with a female twin.

Those with male twins also had a slightly higher rate of excess fats and cholesterol - 11 percent versus 10 percent - but did not have higher rates of type 2 diabetes.

When the authors divided the participants into groups of women that were less than 60 years old and those 60 and up, they found that the difference in weight was restricted to the older women. The difference in cholesterol levels remained for both age groups.

Kristen Culbert, a graduate student at Michigan State University in East Lansing who studies the effects of being an opposite sex twin on metabolism and behavior, said the findings were "interesting," but mostly relevant to researchers.

Babies' brain development is influenced by the hormones they are exposed to in the womb. Researchers believe that sharing the womb with a male twin might mean that females are exposed to greater amounts of predominantly male hormones, called androgens, than those without a male twin.

"The effect is small," said Culbert, who was not involved in the study, and "it's probably not acting in isolation." What hormones a baby is exposed to, she explained, is just one of the many factors that influence weight and disease risks.

Another explanation Eriksson and his colleagues cite is that the difference between growing up with a brother and growing up with a sister may affect women's weight and their risk of dyslipidemia - however, it's unclear why some of those differences wouldn't show up until middle age.

If hormone exposure in the womb is partially responsible for these differences, it could be that changes in hormone levels during menopause mean that the effects of early hormone differences don't show up until after menopause, the authors said.